I can't understand why a business interested in making money and word of mouth would not follow up a wrong account number with an e-mail. I'm guessing a dud number is more likely a mistake than an attempt at fraud. It would make sense to check it out.

I am always surprised at tourist businesses that treat people as a one-off profit center. I love the look on peoples faces when they see me in their place for the second, third and fourth times! They know each time they see me, the spending begins!

Note to Antonio-
I have used the online system that some of the Madrid hostals (including Playa) provide for making reservations and it does warn that reservations will be cancelled if the number isn't valid. I know that the credit card machine we have in the place I work has a function for pre-authorizing charges. Whether they used that I don't know. I always confirm with a follow up e-mail to avoid this.